Synthetic mineral sand produced from staurolite

ABSTRACT

There is provided a synthetic free flowing sand characterized by minimal and irreversible thermal expansion at temperatures up to 2675*F., the sand being the product resulting from calcination of a predominantly staurolite mineral concentrate at a temperature of 2100*F. to 2300*F. for a sufficient time to convert essentially all of the alumina content thereof to mullite. Preferably the sand will have an AFS fineness of 50-80. The sand is suited to various uses including the manufacture of foundry molds.

United States Patent Garnar, Jr.

SYNTHETIC MINERAL SAND PRODUCED FROM STAUROLITE [75] Inventor: Thomas Edward Garnar, Jr., Swan Lake, Fla.

[73] Assignee: E. I. du pont de Nemours and Company, Wilmington, Del.

[22] Filed: Oct. 17, 1972 [21] Appl. No.: 298,303

[52] 11.8. C1. 106/288 B, 106/65 [51] Int. Cl C08h 17/04, C09c 1/40 [58] Field of Search 106/288 B, 65

[56] References Cited UNITED STATES PATENTS Svikis 106/65 3,128,194 4/1964 Christie 106/65 Primary ExaminerDelbert E. Gantz Assistant Examiner-J. V. Howard 57 ABSTRACT There is provided a synthetic free flowing sand characterized by minimal and irreversible thermal expansion at temperatures up to 2675F., the sand being the product resulting from calcination of a predominantly staurolite mineral concentrate at a temperature of 2100F. to 2300F. for a sufficient time to convertessentially all of the alumina content thereof to mullite. Preferably the sand will have an AFS fineness of 50-80. The sand is suited to various uses including the manufacture of foundry molds.

4 Claims, 1 Drawing Figure HOLLOW OONFIIIEO EXPANSION (EQUILIBRIUI) III/IN.

PAIENIEII 3,860.435

LEGEND A 0.000

FLORIDA ZIR 2 CALCINED 5T LITE 0.055 3 STAUROLIIE 4 SILICA I000 I200 I400 I600 I000 2000 2200 2400 TEIIP. F

SYNTHETIC MINERAL SAND PRODUCED FROM STAUROLITE BACKGROUND OF THE INVENTION Staurolite 80% Tourmaline 8% Titanium Minerals 3% Aluminum and Zirconium Silicates 4% Quartz 5% A typical chemical composition (weight basis) is as follows:

A1 47% F620: 13% ZrO 2% TiO, 3% Free Silica 4% 3O Owing to its relatively low cost and an attractive combination of physical and chemical properties, staurolite is finding increasing acceptance as a specialty sand for foundry molding methods. While indeed it has been found to be satisfactory in certain casting operations, experience has further shown that with metals requiring pouring temperatures above 2100F., the staurolite undergoes violent expansion which, in the case of grey iron for example, gives rise to excessive penetration of the mold.

SUMMARY OF THE INVENTION In accordance with the invention it has been found that a synthetic free flowing sand can be fabricated from a predominately staurolite mineral concentrate which, at a temperature even up to temperatures of 2675F. is characterized by minimal and irreversible thermal expansion. This novel sand product is obtained by calcining the staurolite concentrate at a temperature of 2100F. to 2300F., preferably at a temperature of 2150F. to 2300F., for at least a sufficient time to convert essentially all of the alumina content of the staurolite to mullite.

The invention will be further understood by reference to the Drawing wherein there are plotted the thermal expansion characteristics for the said novel product as well as those for uncalcined staurolite, silica and Florida zircon. Values of expansion (plotted along the abscissa) are in units of inches per inch and are measured by the hollow confined expansion test (Section 10, Foundry Sand Handbook, 7th Edition, 1963, published by American Foundrymens Society, Des Plains, Illinois) using discs formed by ramming a sample of sand (containing a small quantity of Western bentonite as a binder) to about 40 percent compressibility and green compressive strength of 10-16 psi.

Thus referring to the Drawing it will be observed that silica, a widely used foundry molding sand, undergoes considerable expansion until, as is known, a phase change from alpha to beta occurs at l063F. But the phase change is reversible such that upon cooling a sample undergoes inversion and will shrink. Thislack of stability is therefore a severe limitation upon the use of silica sand in many areas of the foundry industry.

As further shown in the Drawing, Florida zircon is relatively highly stable, and this has led to its wide acceptance as a foundry sand. It is, however, considerably higher in cost than silica since in the Western 'Hemisphere it is obtainable only from ores containing low concentrations of this mineral.

A third plot in the Drawing demonstrates the violent expansion which occurs at higher temperatures upon heating an untreated, predominately staurolite mineral concentrate. It is this characteristic which tends to limit the utility of staurolite in the casting of grey iron or other metals requiring high pouring temperatures.

The fourth and final plot in the Drawing describes the expansion characteristics of the novel calcined product of the invention. As will be observed, the expansion is minimal, i.e., less than 5 percent, as it gradually increases with increasing temperatures with no marked increase in the range of 2100F. to 2300'F., as experienced with untreated staurolite. The small amount of expansion is, unlike the case with silica, irreversible in that there is no inversion and hence no size reduction to occur upon cooling.

As above indicated, the novel sand product of the invention is obtained by calcining a predominately staurolite mineral concentrate at a temperature of 2100F. to 2300F. The calcination should be carried out for sufficient time to convert essentially all of alumina content of the concentrate to mullite. Completion of the phase change, which can be readily followed by conventional X-ray powder diffraction techniques, is time/temperature dependent such that it occurs more rapidly in the higher part of the temperature range than in the lower part. At a temperature of 2050F.,'virtually no phase change occurs. At a temperature of 2100F., the phase change begins although the X-ray bands oof staurolite do not completely disappear after 15 minutes heating. Operation within the range of 2l50F. to 2300F. is preferred commercially since at that temperature the change occurs very rapidly.

The grains of a staurolite concentrate will typically tend to fuse, sinter, or otherwise adhere to one another Y if a mass thereof is heated above 2 1 00F. Hence to pro duce the product of the invention it is the usual practice to regrind the calcined mass in order to produce abulk product of free flowing characteristics. Other heating techniques can be used however, e.g., a fluidized calcining process, which will provide sufficient spacing between individual grains to reduce or avoid altogether such adherence. It will be understood that a small percentage of loosely aggregated material is not detrimental as such aggregates are readily broken up in the usual handling techniques involved in the preparation of molds.

When intended for use as a foundry sand, it is advantageous that the novel product of the invention have an AFS number in the range of to 80. Values in this preferred range can be achieved by grinding the calcined material to a desired degree. For other uses, the particle size can vary from a fine powdery sand up through the size range commonly attributed to sands.

The product of the calcination step is composed largely of mullite, i.e., 3Al O .2SiO with lesser amounts of a-quartz, and iron compounds as oxides and/or silicates. Other trace components such as TiO and ZrO may also be present depending upon their occurrence in the original staurolite. It will be noted that while no major change appears to occur in the empirical formula as a result of the calcination step, the final product can no longer be considered to be a staurolite mineral.

The novel sand product of the invention is characterized by grains of improved packing characteristics as they tend to be considerably more round than those of the origial staurolite concentrate. It is also significant that the sintering point of the product has been increased from a value of about 2240F. for staurolite to that of 2675F. as a result of the calcining step. Because of these and other unique properties, it will be understood that while the product of the invention has been described as being particularly suitable for foundry molding applications, it is also useful for other pur poses, for example, as a burnishing grain or sandblast abrasive.

EXAMPLE 1 A sample of commercially available staurolite was obtained having a typical mineral composition as given hereinbefore, i.e., containing approximately 80 percent by weight of staurolite with the remainder constituting other minerals. It had an AFS fineness number, at room temperature, of 72.2.

The staurolite sample was placed in a glass vessel and heated for minutes in an oven maintained at a temperature of 2300F. After removal from the oven the sample was cooled, screened and found to have an AFS fineness of 57.4. The stability of the calcined sand product was demonstrated by reheating it to a temperature of 2300F. for another 10 minute period. At the end of this time the AFS fineness was virtually unchanged, being 56.1, thus indicating that virtually no alternation of dimension has taken place. This characteristic indicates that the calcined sand product is highly satisfactory as a foundry molding sand.

EXAMPLE 2 Three tons of commercially available staurolite was obtained, having typical mineral composition as given hereinbefore, i.e., containing approximately 80 percent by weight of staurolite with the remainder constituting other minerals. It had an AFS fineness number at room temperature of 74. The staurolite was fed to an 18 inch internal diameter fluid bed reactor of commercial design at the rate of 120 lbs./hour. By means of preheated air as a fluidizing gas, the bed temperature was maintained between 2l40F. and 2200. with an average 2l95F., with bed depth of4 feet and air rate 79 SCFM. Approximately one ton of staurolite sand was calcined over a period of 16 hours with no serious agglomeration taking place in the bed. The grains in the resulting product expanded. After cooling, the sample was screened and found to have an AFS fineness number of 65.4. The stability of the calcined sand product is demonstrated by the hollow confined expansion curve shown in the Drawing. This shows the calcined product is nearly equivalent to zircon, one of the most refractory sands available. This characteristic indicates that the calcined sand product is highly satisfactory as a foundry sand.

What is claimed is:

l. A synthetic free flowing sand suitable for use in the manufacture of foundry molds and characterized by minimal and irreversible thermal expansion at temperatures up to 2675F., said sand being the product resulting from calcination of a predominantly staurolite mineral concentrate in a fluid bed reactor at a temperature of 2100F. to '2300F. for a sufficient time to convert essentially all of the alumina content of said staurolite to mullite.

2. A sand according to claim 1 having an AFS fineness of 50 to 80.

3. A sand according to claim 1 wherein the calcination has been effected at a temperature of 2l50F. to 2300F. I

4. A process for producing a synthetic free flowing sand suitable for use in the manufacture of foundry molds and characterized by minimal and irreversible thermal expansion at temperatures up to 2675F. which comprises calcining a predominantly staurolite mineral concentrate in a fluid bed reactor at a temperature of 2l00F. to 2300F. for a sufficient time to convert essentially all of the alumina content of said staurolite to mullite. 

1. A SYNTHETIC FREE FLOWING SAND SUITABLE FOR USE IN THE MANUFACTURE OF FOUNDRY MOLDS AND CHARACTERIZED BY MINIMAL AND IRREVERSIBLE THERMAL EXPANSION AT TEMPERATURES UP TO 2675*F., SAID SAND BEING THE PRODUCT RESULTING FROM CALCINATION OF A PREDOMINANTLY STAUROLITE MINERAL CONCENTRATE IN A FLUID BED REACTOR AT A TEMPERATURE OF 2100*F. TO 2300*F. FOR A SUFFICIENT TIME TO CONVERT ESSENTIALLY ALL OF THE ALUMINA CONTENT OF SAID STAUROLITE TO MULLITE.
 2. A sand according to claim 1 having an AFS fineness of 50 to
 80. 3. A sand according to claim 1 wherein the calcination has been effected at a temperature of 2150*F. to 2300*F.
 4. A process for producing a synthetic free flowing sand suitable for use in the manufacture of foundry molds And characterized by minimal and irreversible thermal expansion at temperatures up to 2675*F. which comprises calcining a predominantly staurolite mineral concentrate in a fluid bed reactor at a temperature of 2100*F. to 2300*F. for a sufficient time to convert essentially all of the alumina content of said staurolite to mullite. 